<details>
    <summary>我是标题</summary>
    <div class="content">我是内容
        <br>
        sdfasdfsdf
        <br>
        sdfasdfsdf
        <br>
        sdfasdfsdf
        <br>
        sdfasdfsdf
        <br>
        sdfasdfsdf
        <br>
        sdfasdfsdf
    </div>
</details>
<style>
    details {
        position: relative;
    }

    summary {
        cursor: pointer;
        user-select: none;
    }

    .content {

        opacity: 0;
        overflow: hidden;
        transition: opacity 1.5s ease;
    }

    details[open] .content {
        opacity: 1;
        background-color: red;

    }
</style>